Forearc Sedimentation, Subduction Erosion and the Effect of Seamount Collisions in the Tonga Arc System

1996 American Geophysical Union Fall Meeting

P Clift (WHOI, Woods Hole, MA, 02543)

C.J. MacLeod (Dept. of Earth Sciences, University of Cardiff, PO Box 914,Cardiff CF1 3YE, UK)

D. Tappin (British Geol. Surv., Keyworth, Nottinghamshire, NG12 5GG, UK)

D.J. Wright and S.H. Bloomer (both at: Dept. of Geosciences, Oregon State University, Corvallis, OR 97331-5506)

Sedimentation in the Tonga Forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front through mass flows and turbidity currents on to the adjacent Tonga foerarc basin (Tonga Platform), with the greatest thicknesses accumulating close to the arc itself. Erosion of the Tonga Platform along its trenchward edge has resulted in the development of canyons and the redeposition of material down slope into small, perched basins, measuring approximately 5 km by 15 km, formed on the back of tilted normal fault blocks in the mid slope area, especially between 18S and 20'S. Outside these centers sedimentation on the trench slope is slow, resulting in Mn crust formation and localized mass wasting in the form of talus near fault scarps. Around 18S and 23-25S, which are areas of recent seamount collision, the trench slope is markedly steeper and such basins are less well developed. Consequently mass wasting of arc and platform debris extends deep into the trench.

Collision of the Louisville Ridge with the trench appears to have caused modest (< 300m), temporary uplift of the Tonga Platform at the point of collision. However, on the trench slope collision resulted in compression, greater uplift and arcward tilting of the basement, generating unconformities within the mid-slope terrace basins. Subsequent collapse of the forearc basement and resumption of extension results in the deposition of a new stratigraphic sequence unconformably overlying the older series, as recorded at ODP Site 841 during the middle Miocene. Rapid late Eocene subsidence of the mid-slope terrace at ODP Site 841, following subduction initiation in the mid Eocene, may be due to the formation of the trench at that time. Extensional unroofing of the arc basement during the formation of the trench resulted in the erosion of arc lower crustal dunite and mantle peridotite into mass flow conglomerates. Post-early Oligocene subsidence reflects slow, continuous subduction erosion of the outer forearc, with < 20-30 km being eroded from the plate margin since that time.

Return to Dawn's Vita